Natural gas is a vital source of heat energy in the United States. Its selling price is based on volume and heat content. The heat content is greatly influenced by the presence of the heavy (higher molecular weight) components. These heavy components also have a large influence on the gases physical properties, which in turn impact flow rate calculations. The heat content and physical properties of gas are primarily determined by calculations based on gas composition.
The gas composition is determined by analysis on gas obtained by one or more of three means: spot sampling, composite sampling, and on stream (on line) analyzer sample systems. Spot sampling consists of extracting a natural gas sample at a point in time representing source gas composition at that point in time. The sample is stored in a sample container (sample cylinder).
Composite sampling consists of extracting very small increments of natural gas samples over a long period of time, usually one month, said samples being stored in a single sample container. The result is a composite of a sample gas representing the entire quantity of source gas, which flowed through the pipeline during the sampling period. Sample containers containing stored spot or composite samples are transported to a laboratory and analyzed for composition.
In the case of “on stream analyzer sample systems” a sample is withdrawn continuously or semi-continuously and routed directly into to an analyzer, usually a gas chromatograph, for real time analysis. It is well known by the natural gas industry that it is very difficult to obtain a representative sample of a natural gas stream, which is at or below its hydrocarbon dew point temperature (H.C.D.P.).
The American Petroleum Institute (API) Manual of Petroleum Measurement Standards, Chapter 14, Section 1 and Gas Processors Association's (GPA) Standard 2166, “Obtaining Natural Gas Samples for Analysis by Gas Chromatography”, are good references for sampling of natural gas. Both organizations recognize the difficulty in sampling natural gas with high H.C.D.P. The GPA Standard 2166 outlines eight methods for spot sampling with sample cylinders. The API 14.1 standard provides further recommendations on the use of these methods.
The standards refer to several sources of error. Some of the most important are as follows:
                1) Sampling a natural gas source containing liquid in any form. i.e.—The source gas is at or below its H.C.D.P. temperature.        2) Sampling a natural gas source at an ambient temperature that is below the H.C.D.P. temperature of the flowing gas source.        3) Throttling of the sample gas stream as it flows from the source to a sample container or on stream analyzer, especially if the throttling causes the temperature to drop (Joule-Thomson cooling effect) to near or below the gas H.C.D.P.        
The importance of proper treatment of natural gas samples that are near or below their H.C.D.P. is the focus of the industry's attention, especially in light of the mining of natural gas from deeper reserves having higher H.C.D.P. temperatures and the increasing value of natural gas.
Major problems with spot and composite sampling are the accumulation of liquid in the sample container during the “purging” phase and depletion or accumulation of high molecular weight components during the sample filling phase.
During the purging phase of sampling, sample lines from the source to the sample container and the sample container itself are purged with sample gas to remove or displace residual gases. Most of all of the problems addressed by the eight GPA Sampling methods are related to the purging phase.
There are currently two basic types of sample containers (sample cylinders). One is the constant volume type, which is usually a small steel cylinder with fixed volume having valves at one or both sides. The second type is a constant pressure sample cylinder. This is a small steel cylinder having an internal “floating” piston, end caps at both cylinder ends, and valves in both end caps. The floating piston separates the internal cylinder volume into two cavities. The pressure in the two cavities is maintained at somewhat the same level by movement of the floating piston. For example, if the pressure in one cavity is raised above that of the other cavity, the floating piston is moved in the direction of the lower pressure cavity until the pressure in both cavities is somewhat equilibrated. This type of sample cylinder was designed to prevent “throttling” of sample gas.
After purging of exterior lines, pre-charge gas (gas stored at elevated pressure in one of the cavities before actual sampling begins) is released slowly. When the “pre-charge” gas pressure drops below the sample supply pressure, the piston moves towards the pre-charge cavity end of the cylinder allowing the sample gas to enter the sample cylinder without throttling.
Several problems are associated with the use of this type of hardware. One such problem is that the friction of the piston seals can cause a pressure difference of 20 to 30 PSI between the two cylinder cavities. The second is that the piston seals can harbor contaminates from previous samples. Cleaning is very difficult and time consuming. The entry ports and valving designs in the end caps require purging and also represent a source of sample composition distortion during the purging phase.
Constant volume cylinders are difficult to clean between samples as recommended by API and GPA standards. It is also difficult if not impossible to verify if they are indeed clean after the cleaning process.
Further, prior art valves typically installed on sample cylinders are prone to becoming damaged during transportation. The valve knobs are mounted on slender stems, which protrude from the valve body. The stems are often bent when cylinders are dropped or handled roughly. Valve stem packing leaks are also a common problem. Vibration and rough handling during transportation an also result in valves becoming partially opened thereby allowing fluids to leak out.
The prior art has therefore failed to provide a constant volume-type sample cylinder which is easy to clean and service, reliable in operation, and effective in preventing throttling of the sample gas.